1. Field of the Invention
The present invention relates to a method and device for detecting an amount of particulate matter contained in a gas to be measured, such as an exhaust gas from vehicles, and in particular, the method and apparatus detect an amount of the particulate matter based on electrical resistance caused by the particulate matter accumulating between electrodes. The present invention also relates to a method of manufacturing a particulate matter detection element used in the particulate matter detecting device.
2. Description of the Related Art
The exhaust gas of a diesel engine of an automobile and the like may include environmental pollutants, particularly particulate matter (hereinafter referred to accordingly as “PM”) mainly composed of soot particles and soluble organic fractions (SOF). A diesel particulate filter (hereinafter referred to accordingly as “DPF”) is provided on an exhaust gas path to collect the PM. The DPF is made of a porous ceramic having excellent heat resistance. The DPF captures the PM as a result of the exhaust gas passing through a partition wall having numerous fine pores.
When the amount of collected PM exceeds an allowable amount, the DPF becomes clogged. Pressure loss may increase. Alternatively, the amount of PM escaping through the DPF may increase. Therefore, collection capability is recovered by a regeneration process being periodically performed.
In general, increase in the differential pressure across the DPF caused by increase in the amount of collected PM is used for determining the regeneration timing. Therefore, a differential pressure sensor is provided that detects the difference in pressure upstream and downstream from the DPF.
The regeneration process is performed by high-temperature exhaust gas being introduced into the DPF through heating using a heater, by post-injection, or the like, and the PM being removed by burning.
On the other hand, a sensor capable of directly detecting the PM in the exhaust gas has been proposed. The PM sensor is, for example, provided downstream from the DPF and measures the amount of PM escaping through the DPF. The PM sensor can be used in an on-board diagnosis (OBD) device to monitor an operating state of the DPF or to detect abnormalities such as cracks and damage.
Use of the PM sensor in place of the differential pressure sensor to determine the regeneration timing of the DPF is also being discussed. In this instance, the PM sensor is provided upstream from the DPF and measures the amount of PM entering the DPF.
As a basic configuration of a PM sensor such as that described above, JP-A-S59-197847 discloses an electrical-resistance-type smoke sensor. The smoke sensor is configured such that a pair of conductive electrodes are formed on a front surface of a substrate having insulating properties, and a heating element is formed on a back surface of or within the substrate. The smoke sensor takes advantage of smoke (particulate carbon) having conductivity, and detects changes in electrical resistance value occurring as a result of smoke accumulating between the electrodes that serve as a detection section.
In a particulate matter detecting device such as that described above, when a certain amount of particulate matter or more is accumulated between detection electrodes, the detected resistance no longer changes. The amount of particulate matter within gas to be measured can no longer be detected.
Therefore, the heating element that generates heat as a result of being energized is provided. The detection section is heated by being directly heated by a heater. Alternatively, the detection section is heated by post-injection or the like, by exhaust gas, serving as the gas to be measured, being heated to a high temperature. As a result, the particulate matter accumulated between the detection electrodes is removed by burning. Detection capability is thereby recovered.
In addition, WO 2008/031654 discloses an example of a particulate matter detection element, such as that described above, and a control method. The particulate matter detection element in WO 2008/031654 is configured such that a resistance layer is connected in parallel to electrical resistance formed by particulate matter accumulated between detection electrodes. The resistance layer is provided between a substrate and a pair of detection electrodes. The resistance layer is formed by a conductive layer containing zirconia or the like. As a result of the resistance layer being formed, damage and deterioration of the electrodes can be detected.
However, in a conventional electrical-resistance-type particulate matter detecting device, such as that described in JP-A-S59-197847, when the particulate matter accumulated between detection electrodes is heated and removed, the resistance between the detection electrodes becomes extremely high, causing an almost insulated state.
Therefore, it may be difficult to use the value of the detected resistance to differentiate between a state in which particulate matter is not accumulated between the detection electrodes, and a state in which a disconnection abnormality has occurred in an electrical wire of a signal line connecting a detection element and a detection circuit or the like.
As described in WO 2008/031654, energization is performed between the detection electrodes by the conductive layer. Therefore, output is detected even in a state in which particulate matter is not accumulated, if the resistance value of the conductive layer is too low. As a result, malfunction may occur. Furthermore, the resistance value of the conductive layer is required to be adjusted with high accuracy, leading to increase in manufacturing cost.
Moreover, the metal configuring the detection electrodes inevitably becomes dispersed in the conductive layer as a result of extended use. The resistance value of the conductive layer changes, thereby causing instable output.